Demands for a copying machine and a laser printer are increasing more and more in response to wide distribution of Personal Computers (PCs) and office automation. Both the copying machine and the laser printer are an image forming apparatus that forms a desired image on a printing paper by transferring toner thereon, and thus essentially uses toner to form an image.
Along with the increasing demand of the market, consumer requirements for the copying machine, the laser printer and the like are gradually becoming stricter. Examples of such requirements include clearer image quality, durability ensuring that toner will show no deterioration in charging characteristics even if it has been used for a long time period, the miniaturization of the copying machine or the printer, low price, high printing speed, energy saving, easy recyclability and the like.
Of the above requirements, durability is required for toner itself. That is, durability ensures that a clear image can be continuously maintained and charging characteristics will not degrade. In the fields of producing toner, researches are being conducted in various aspects in order to produce durable toner.
Toner is a developer material that the printer or the copying machine, as mentioned above, uses to form an image on an image receptor in a transfer operation. In order to produce durable toner, which can continuously maintain a clear image, processes of using toner in the copying machine or the laser printer should be understood first of all.
An image forming apparatus, such as a copying machine or a laser printer, which produces printouts by transferring toner, generally carries out a printing process as follows:
1. First, a charging step of uniformly charging the surface of a drum is performed. The drum is generally implemented with an Organic Photo Conductor (OPC) drum and the like. The charging is conducted by electro-statically charging the surface of the drum using a charging rayon brush or the like.
2. An exposure step of forming a latent image by exposing the surface of the drum is followed. A conductor such as an OPC on the evenly-charged surface of the drum is an insulator when light is not incident thereon, but acts to conduct charges in the presence of light. Thus, when the drum surface is exposed to a beam for example from a laser, the portion exposed to the beam is discharged or neutralized.
3. Separately to the exposure step, a step of attracting toner to the surface of a developing roller is carried out. This step is a preliminary step, followed by a step of forming a toner image on the charged drum.
4. Then, the step of developing a toner image on the surface of the drum using toner, attracted to the surface of the developing roller, is performed. As mentioned above, when the drum surface is exposed to light, the exposed portion thereof is discharged or neutralized. This is because, when toner is charged with the same polarity as that of the drum, the surface of the drum, if not exposed to light, will repel toner, thereby preventing toner from migrating thereto. However, the exposed portion of the surface of the drum does not repel toner, so that toner can adhere to the latent image, thereby forming the toner image.
5. The developing step is followed by a step of transferring the toner image from the drum surface to an image-receiving paper (i.e., a printing paper). In the transferring step, the surface of the image-receiving paper is charged with a polarity opposite to that of toner in order to generate an attraction force for toner, and the drum and the image-receiving paper are placed adjacent to each other in order to facilitate the transferring.
6. Since toner is not permanently bonded to the image-receiving paper even though it is transferred to the image-receiving paper, a step of fusing toner to the image-receiving paper is followed. The fusing step is completed generally by allowing the image-receiving paper, on which the toner image is formed, to pass through a pair of rollers including a heat roller and a pressure roller, so that toner is compressed by heat and pressure and a binder contained in toner forms a coating layer around toner.
7. Finally, a step of cleaning residual toner from the surface of the drum prior to the recharge of the drum, so that the drum can be charged again for the next operation.
In consideration of the above-mentioned printing process, basic characteristics of toner required for respective steps of the printing process can be understood.
First, it is necessary that toner have at least a predetermined charge amount, so that toner can adhere to the developing roller, then be developed on the OPC drum, and then be transferred to the image-receiving paper. That is, since toner is charged by friction against a doctor blade in the process of adhering to the developing roller in a toner hopper of a toner cartridge, it is required that toner be charged with a pre-determined amount or more, so that the subsequent steps, such as migration from the developing roller to the charged drum and transferring from the charged roller to the image-receiving paper, can be easily carried out.
It is also required that toner, after being charged, continuously maintain the charge state before being transferred to the image-receiving paper. This is referred to as charge-maintaining ability that can prevent the charge from being lost through contact with a conductive material or another toner and also ensure that toner maintain high chargeability.
There are also required some properties such as excellent transfer property, low temperature fusion ability and anti-offset property. With the excellent transfer property, toner can be easily transferred from the photoconductive drum to the image-receiving paper. The low temperature fusion ability allows toner to be easily fused even if it is not heated to a high temperature in the fusion. The anti-offset property can excellently resist against the offsetting of residual toner to the surface of the charged roller. In particular, the transfer property is greatly dependent upon the sphericity of toner, and thus, when the shape of the particles is irregular, a scattered laser beam may degrade the transfer property. That is, when color correction is carried out in the printer, the image density of an OPC drum is measured, followed by toner correction according to colors to be most similar to an actual image. If toner is completely spherical, color correction is made completely when the image density is measured. However, as the shape of toner deviates more from the spherical shape, the scattering of the laser beam will more degrade the correction, and thus a clear image cannot be produced. Furthermore, when the sphericity increases, the adhering force of toner is also enhanced to increase the migration of toner from the drum to a transfer belt, thereby improving the transfer property.
In addition, other properties such as cleaning performance and anti-contamination property are required.
In particular, recently, the above-mentioned properties are required complexly and comprehensively owing to the increased necessity of high image quality, high speed and color expression.
Accordingly, in order to satisfy all of the above requirements, a toner generally includes toner particles, which include a colorant, a binder resin, a wax, a dispersant, a charge control agent and the like, and an outer additive adhering to the outside surface of the toner particles.
The binder resin is melted by heating during the fusion of the toner to help the toner adhere to the surface of an image-receiving paper. The wax makes an image glossy after being printed while dropping the melting point of the toner particles. The dispersant induces uniform dispersion, and the charge control agent is used to control the charge of the surface of the toner particles.
Of these additives, because of the charge control agent (abbreviated “CCA”) of the toner particles, the surface of the toner particles can be charged when the toner is in friction with a doctor blade. It is required that the CCA be dispersed as evenly as possible on the surface of the toner particles.
The CCA generally tends to form a phase separated from the binder resin. In this case, the toner in friction with the doctor blade is not evenly charged, and charge distribution is made very wide due to uneven fiction. Accordingly, respective toner particles have different charges, and thus are not suitably used in a printing, which is performed based upon the adjustment of the charge,
Separately from the above, the toner particles are generally produced by a pulverization process of melting the above-mentioned components, forming a sheet material from the melt, and mechanically pulverizing the sheet material, or by a polymerizing process. The former process of mechanical pulverization is widely used up to present since it is relatively easier to produce toner. However, in the case of producing the toner particles by the mechanical pulverization, the toner particles come to have irregular shapes and a large amount of cracks exist in the surface of the toner particles. The problem of this process is that the radiation of light such as a laser beam causes very severe scattering. Accordingly, for the toner particles produced by the mechanical pulverization, it is required to overcome a problem of reduced transfer property.
As the printing speed of the printer changes among low, middle and high speeds, the charging characteristics of toner also change. When toner is charged without the consideration of the charging characteristics, an excessive charge may cause a background contamination such as tire tracking or an insufficient charge may produce an uneven image. Accordingly, it is required to provide toner products, which have a specific charge suitable for a respective printing speed. However, such toner products have not been developed up to the present.